A preformed sheet is known from WO2007003334. This reference describes a preformed sheet comprising at least two mono-layers, each mono-layer preferably containing unidirectionally oriented high strength fibers having a tensile strength of at least 1.2 GPa, and preferably a binder, with a fiber direction in each mono-layer being rotated with respect to the fiber direction in an adjacent mono-layer, and a separating film on at least on one of its outer surfaces, wherein the separating film has an areal density of between 1 and 5 g/m2.
A drawback of the preformed sheet known from the prior art is that the ballistic protection against 9 mm bullets, expressed as the 9 mm V50 of a ballistic-resistant article comprising said sheets, after immersion or soaking in a liquid such as JP8 fuel as well as in seawater, significantly degrades. For certain applications a higher retention of ballistic properties after immersion in JP8 fuel (jet fuel specified by the standard MIL-DTL-83133) and also in seawater is of great importance.
US2009/0025111 describes a flexible ballistic resistant composite material that has improved resistance to pick-up of water and other liquids, the composite material comprising a plurality of non-woven fibrous layers, the fibrous layers comprising a network of high tenacity fibers selected from the group consisting of aramid fibers, extended chain polyethylene fibers, rigid rod fibers and blends thereof, the fibers being in a binder comprising a thermoplastic polyurethane resin. Still, the 9 mm V50 retention after immersion in JP8 fuel or seawater could be improved.
US2011/0143086 describes a ballistic resistant fabrics and articles that retain superior ballistic resistance performance after exposure to liquids such as seawater and organic solvents, such as gasoline and other petroleum-based products. The fabrics are formed from high performance fibers coated with a nitrile rubber binder polymer having an acrylonitrile content of from about 15 wt. % to about 50 wt. %. However, the processing of nitrile rubber binders is extremely difficult and negatively related to safety, environment and health issues.
There is thus a constant need in industry for a preformed sheet that enables making of ballistic-resistant articles offering a higher protection level after soaking of the article in liquids such as JP8 fuel and sea water, or alternatively for a preformed sheet that enables making of ballistic-resistant articles with a higher retention of the article after immersion or soaking in those media.
According to the invention, this is provided by a preformed sheet, wherein the separating film has surface tension of less than 35 mN/m at the surface faced to the fibrous network and a crystallinity of between 50 and 90%.
A preformed sheet according to the invention enables making of ballistic-resistant articles with a higher retention of the article after soaking in JP8 fuel and seawater. With “retention after soaking in JP8 fuel and seawater” in this application is meant the retention after soaking in JP8 fuel as well as after soaking in seawater.
With ballistic-resistant articles are meant shaped parts, comprising an assembly of at least two preformed sheets according to the invention or just one preformed sheet according to the invention, which can be used as, for example, protective clothing or for armouring of vehicles, and which offer protection against ballistic impacts such as by bullets and ballistic fragments.